⚡ Bolt: optimize Sprite component rendering and asset loading

[b0x1]

This PR implements a set of systemic performance optimizations for the Sprite component, which is used extensively for rendering tiles, units, and resources.

💡 What:

1. Memoization: Wrapped the Sprite component in React.memo to skip re-renders when props haven't changed.
2. Computation Caching: Moved calculateSheetSize outside the render loop and used a WeakMap to cache results. Previously, this $O(N)$ calculation (where $N$ is the number of frames in the manifest) happened on every single render.
3. Fetch Coalescing: Added a pendingManifests cache to ensure that if multiple sprites request the same manifest (e.g., during map loading), only one fetch call is made.
4. Cleanup: Standardized constants and fixed linting issues.

🎯 Why:
The Sprite component is a high-frequency leaf component. In screens like the Settlement Screen or during map panning, hundreds of Sprite instances can be active. Reducing the per-instance overhead significantly improves UI responsiveness and reduces CPU/Network spikes.

📊 Impact:

• Eliminates redundant $O(N)$ manifest parsing on every render.
• Reduces network overhead by deduplicating manifest fetches.
• Prevents unnecessary React reconciliation for static sprites.

🔬 Measurement:

• Verified that game rendering remains correct via Playwright screenshots.
• Confirmed pnpm run check (Lint/Type/Test) passes.
• Logical complexity of derived state calculation reduced from $O(N)$ to $O(1)$ cached.

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PR created automatically by Jules for task 2004112129294971924 started by @b0x1

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